1. Field of the Invention
The present invention relates to a control system for a vehicle having a continuously variable transmission capable of changing a gear ratio continuously and, more particularly, to a control system for controlling a power source and a continuously variable transmission in parallel on the basis of an output demand.
2. Related Art
In the prior art, a continuously variable transmission (i.e., CVT) is known as a vehicular transmission. In short, this continuously variable transmission is a transmission having a function to change the ratio between an input speed and an output speed continuously. At present, there has been practically used a system which is constructed to set the gear ratio properly by continuously changing the radial position of a transmission member on an input side rotary member and the radial position of the transmission member on an output side rotary member. Specifically, there have been practiced the belt type continuously variable transmission and the toroidal type (or traction type) continuously variable transmission.
The former belt type continuously variable transmission is constructed such that an input pulley and an output pulley running over a belt are made of a fixed sheave and a movable sheave which is made movable in the axial direction, respectively, and such that the groove widths of the individual pulleys are changed by moving the movable sheaves of the individual pulleys axially thereby to change the belt winding diameters, i.e., the gear ratios continuously. By applying an axial force according to a demanded transmission torque to the movable sheave in the output side pulley, the tension of the belt is set to one according to the demanded torque. To the movable sheave of the input side pulley, on the other hand, there is applied the axial force for setting the gear ratio which is determined on the basis of an output demand signal based on the depression stroke (or the accelerator opening) of an accelerator pedal, an output demand signal from a cruise control for setting the vehicle speed automatically to a predetermined value, and a running state such as the vehicle speed, thereby to change the groove width of the input side pulley.
Thus according to the continuously variable transmission, the gear ratio can be continuously changed to change the speed of the power source such as an engine freely. In the vehicle mounting the continuously variable transmission thereon, therefore, the gear ratio can be controlled not only to satisfy a demanded driving torque but also to run the power source efficiently, i.e., to reduce the fuel consumption.
One example of this control is disclosed in Japanese Patent Publications Nos. 3-72867 (JP-B-3-72867) and 62-110535 (JP-A-62-110535). In this control, firstly a target driving force is determined from the depression stroke of the accelerator pedal and the vehicle speed. Next, a target power, a throttle opening and a target engine speed corresponding to the target driving force are individually determined, and a target gear ratio of the continuously variable transmission is then determined on the basis of the target engine speed and the vehicle speed. Moreover, the engine is controlled to generate a target torque determined on the basis of the target power and the engine speed, whereas the continuously variable transmission is controlled to set a gear ratio for the most efficient engine speed for achieving the target power.
However, the engine and the continuously variable transmission are different in their responses to the control. Specifically, the engine is enabled to change the torque immediately after the intake air and the fuel supply are increased, but the continuously variable transmission takes a considerable time until the target gear ratio is achieved, because the gear ratio is changed by gradually increasing/decreasing the groove widths of the pulleys by the hydraulic control. In response to the depression stroke of the accelerator pedal, however, the target driving force is changed, and the target power is changed. On the other hand, the engine torque is determined by the calculation of (the target power/the engine speed). If the engine speed is brought in a low state due to a delay in the speed change of the continuously variable transmission, the engine torque, as determined by the calculation of (the target power/the engine speed), abruptly rises, and an acceleration shock may occur. Due to an abrupt change in the torque, on the other hand, a torsional deformation may occur in a power transmission line to cause the longitudinal vibration (or the surging) of the vehicle.
When a torque converter is employed in the starting system of the vehicle having the continuously variable transmission, on the other hand, the driving torque at the start may become excessively high. In the fluid type torque converter, for example, a torque amplifying action occurs in a converter region where the ratio (i.e., the speed ratio) between the input speed and the output speed is smaller than xe2x80x9c1xe2x80x9d, the torque to be outputted from the torque converter increases. At the start, the output speed is substantially zero so that the torque ratio or the torque amplification factor takes the maximum value. As a result, the aforementioned controls of the power source and the continuously variable transmission might make the driving torque excessive at the start. In this case, an abrupt acceleration feeling might be given to passengers of the vehicle, or the tire slip might be invited when a road surface has a low xcexc (i.e., friction coefficient) (as on a pressed-snow road, a frozen road or a gravel road). The torque converter as the starting system basically performs the action to augment the starting torque, as described above, so that a similar situation occurs even if the torque converter is not of the fluid type.
In order to eliminate this disadvantage, it is conceivable to perform the so-called xe2x80x9crounding controlxe2x80x9d, in which the target driving force according to the depression stroke of the accelerator pedal is reached, even if calculated, not instantly but gradually. With the control based on the target value which is corrected in the rounding control or set temporarily, the abrupt change in the torque is avoided to eliminate the shock. However, the rounding control delays the achievement of a final target value by reducing the corrected target value so that the control response is deteriorated. If the gear ratio of the continuously variable transmission is controlled on the basis of the target value by the rounding control, on the other hand, the target value of the gear ratio is gradually changed so that the speed changing rate of the continuously variable transmission cannot be maximized. As a result, the intrinsically inferior shift response becomes worse to cause a problem that the physical discomfort for driving the vehicle becomes more serious.
A main object of the present invention is to provide a control system capable of improving the responses of an engine and a continuously variable transmission and the power characteristics of the vehicle and capable of controlling the vehicle without any physical discomfort.
For this object, therefore, the control system of the invention is so constructed for controlling the power source and the continuously variable transmission to a target value based on an acceleration demand, as to make either a correction only for the target value of one of the power source and the continuously variable transmission or a correction different from that for the target value of the other.
According to the invention, specifically, there is provided a control system for a vehicle including: a power source; and a continuously variable transmission capable of changing gear ratios continuously, wherein a target driving force is calculated on the basis of an acceleration demand and a vehicle speed, and a target power of the power source is calculated on the basis of the target driving force, so that the continuously variable transmission may be controlled to achieve the target power, and wherein a target torque of the power source is calculated on the basis of the target driving force so that the power source is controlled to achieve the target torque. This control system comprises corrected target torque calculating means for determining a corrected target torque changing in the course to reach the target driving force, while being suppressed more than the change of the target torque. The control system is constructed such that the gear ratio of the continuously variable transmission is controlled on the basis of the target power and the load on the power source is controlled on the basis of the corrected target torque.
The corrected target torque calculating means can be constructed to include means for determining a corrected target driving force changing while being suppressed more than the change of the target driving force, to calculate the corrected target torque on the basis of the corrected target driving force.
On the other hand, the corrected target torque calculating means can be constructed to include means for determining a corrected target power changing while being suppressed more than the change of the target power, to calculate the corrected target torque on the basis of the corrected target power.
Moreover, the corrected target torque calculating means can be constructed to include means for calculating the corrected target torque on the basis of the target torque.
In any of the foregoing constructions, moreover, the corrected target torque calculating means can be constructed to include means for calculating the corrected target torque so that the change in the corrected target torque may be restricted for a predetermined period in the course of the change.
The controls for determining the individual corrected values of the target torque, the target driving force and the target power are similar to the aforementioned rounding controls. In the continuously variable transmission, therefore, the shift controls are quickly performed with the target power which changes instantly on the basis of the acceleration demand, whereas in the power source, the gently changing controls are made with the rounded target power value, so that an abrupt torque generation is avoided. As a result, the acceleration shock at the time of quickly opening an accelerator can be reduced while keeping the response of the continuously variable transmission proper, so that the vehicle can be controlled without any physical discomfort.
In any of the foregoing constructions, still moreover, the corrected target torque calculating means can be constructed to include means for calculating the corrected target torque by making the suppression degree of the change in the corrected target torque the lower for the larger acceleration demand or the larger control parameter determined from the acceleration demand.
In any of the foregoing constructions, on the contrary, the corrected target torque calculating means can be constructed to include means for calculating the corrected target torque by making the suppression degree of the change in the corrected target torque the lower for the higher changing rate of the acceleration demand or the higher changing rate of a control parameter determined from the acceleration demand.
With this construction, therefore, the degree of suppressing the change in the corrected target torque is set to compensate the torque loss corresponding to the inertia torque, as caused by the rotational change accompanying the acceleration demand. This suppression degree is determined on the basis of the acceleration demand or a control parameter based thereon, or their changing rate, so that the correction of the target torque compensating the inertia loss torque is not fluctuated by the disturbances. As a result, it is possible to prevent the fluctuation of the output torque and the surging, as caused by the fluctuation. On the other hand, the suppression degree is lowered, as the inertia torque is raised. Specifically, as the acceleration demand or a control parameter based thereon, or their changing rates are raised, the change ratio of the corrected target torque is increased, so that the delay in the rotational change due to the inertia force can be prevented to effect the speed change having a high response.
According to the invention, on the other hand, there is provided a control system constructed to comprise lower limit restricting means for restricting a calculating speed of the target torque of the power source to a predetermined lower limit, at least in a portion of a converter region in which a torque converter amplifies the torque and to calculate the target torque on the basis of the calculation speed restricted to the lower limit.
Here, the lower limit is exemplified by 900 rpm.
In the control system of the invention, moreover, the control system is constructed to comprise upper limit restricting means for restricting the target torque to a predetermined upper limit according to an acceleration demand, at least in a portion of a converter region in which the torque converter amplifies the torque and to control a load on the power source on the basis of the target torque restricted to the upper limit.
Here, the upper limit of the target torque can be set by using a value which is prepared in advance in relation to the accelerator opening, such as a correlation map.
According to the control system of the invention, therefore, an excessive torque can be prevented from being generated in the converter region, without being influenced by the target power based on the target driving force or the speed of the power source, so that the abrupt acceleration feeling at the start or the tire slip or the like on the low xcexc road can be prevented to control the vehicle without any physical discomfort.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrations only and are not intended as a definition of the limits of the invention.